Semiconductor integrated circuits are widely used in computer memory. The cost per bit of storage and speed of operation provided by N-channel MOS random access memory (RAM) devices have resulted in wide acceptance thereof in the manufacture of digital equipment, particularly minicomputers. One example of such a device, a 4096 bit RAM, is shown and described in U.S. Pat. No. 3,940,747, filed Aug. 2, 1973 by N. Kitagawa and C-K Kuo, assigned to Texas Instruments, and in Electronics, Sept. 13, 1973, pp. 116-121. A 16K bit RAM is disclosed in copending application Serial Number 682,687 filed May 3, 1976 by N. Kitagawa et al and assigned to Texas Instruments, and in Electronics, May 13, 1976, pp. 81-86.
A semiconductor RAM accepts a multiple bit address from external circuitry, and this address functions to select a specific cell (or cells) within the RAM for writing in data or reading out data. The address is generated by other parts of the system, separate from the RAM; a requirement placed on the circuitry of the RAM is that it responds to timing and voltage levels or logic levels in the address signals which are compatible with the remainder of the system. Often the logic levels in the system are dictated by bipolar or TTL operating voltages, rather than those of MOS devices. The address inputs to the RAM should impose a minimum of current loading on the external circuits, and a minimum of noise or unwanted voltage variations should be generated by the address signal detecting circuitry. The address buffer circuitry should be responsive to the address signals during only a small window in time during the operating cycle of the digital equipment, so that the address signals may change to set up the next access cycle before the existing cycle is completed. Address buffer circuits which function adequately for these purposes are disclosed in pending U.S. Patent Application Serial Number 624,813, filed Oct. 22, 1975, by Redwine and Kitagawa, now U.S. Pat. No. 4,031,415, and Serial Number 716,843, filed Aug. 23, 1976 by Kitagawa and White, both assigned to Texas Instruments, as well as the above-mentioned Sept. 13, 1973 and May 13, 1976 articles in Electronics. Nevertheless, continuing improvement in these factors, particularly speed, is necessary as the cycle time of computer equipment becomes more demanding.
It is therefore the object of the invention to provide an improved circuit for detecting address signals or other logic levels in semiconductor memory devices or the like, particularly a high speed circuit which is compatible in response timing, voltage level, and loading with the remainder of the system in which the device may be used.